We now have a means to evaluate directly the activity of the sympathetic nervous system. Measuring the integrated neural activity from either pre- or postganglionic sympathetic fibers will allow us to determine (1) ganglionic blocking action or (2) inhibition of the sympathetic nervous system by central action or via reflexes. We have synthesized a large number of cyclic dopamine analogs. Many of these agents lower blood pressure and produce bradycardia. We have evaluated the potent ganglionic blocking actions of a limited number of agents. We have compounds that are potential hypotensive agents and we wish to determine whether they are acting on the central nervous system or not. We will be evaluating the mechanism by which many of these agents inhibit ganglionic transmission. We will evaluate whether the mechanism is similar or different from the nerve terminals.